<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        div {
            width: 400px;
            height: 400px;
            background: skyblue;
        }
    </style>
    <script src="jquery-3.2.1.min.js"></script>
    <script>
        $(function () {
            $('#btn').on('click',function () {
                $('div').fadeTo(1000,.3);
            })
        })
    </script>
</head>
<body>
<div></div>
<button id="btn">click</button>
</body>
</html>